Solar Expert Posts: 259 ✭✭
How do I conceptualize this...

With fossil fuels or even hydro, if a plant has a 10MW capacity, then it can produce 10MW every hour. That plant can have water falling through 24hrs a day or burn fuel 24hrs a day. This means that plant has a capacity of 10MW. Great!

Enter renewables such as wind and sun. Wind is quite varied because it depends on where they are installed, they could produce constantly and it is assumed that they are installed in places where there is a 'constant-uninterrupted' wind flow that generates a 'stable' power production. So we could assume that a 10MW wind plant could produce 10MWhrs in an hour of operation and thus be classified as a 10MW plant.

But solar, no matter where its installed, will only generate around 5 hours at best. And im using a whole number to make math easy

So if we determine a solar plant has the installed capacity of 10MW, it can really only produce them during 5 hrs.

So how do solar plants get classified in that sense?

• Banned Posts: 17,615 ✭✭✭

Well you're pointing at the #1 argument against RE: it produces when it wants to, not when you need it.

So your 10 mW plant can produce 240 mW hours in a day.
A 10 mW solar or wind turbine will produce ... whatever it does.
The determination is made based on historic data for wind and solar at the site of installation. In other words "on average we get X MPH winds for Y hours" or "this area receive X hours of equivalent good sun" and multiply by the size of the install. The way we use PV Watts to predict our home GT installation's contribution to the electric bill.

The utilities will (quite rightly) point out this is no guarantee of produce Watt hours. They will also point out it makes grid management more difficult because they can't turn these sources on/off as needed (standard power plants all have a certain ramp-up time depending on the type) the power is only there when it's there. As such they have to change management, give priority to RE, and use conventional generation to make up the difference as needed.

Since I personally know someone very much responsible for grid management I can say I've heard all the anti-RE rants from that side. :roll:
• Solar Expert Posts: 3,123 ✭✭✭✭

They get classified as 10Mw plants in the popular press but have to be classified as 10Mw plants producing unreliable power and only during specific time periods (hopefully the high demand periods by the people responsible for running the power network.

When the statisticians or popular press are trying to determine the percentage of the total generating capacity (or generation) that comes from renewables, they really should be comparing the output per year, in GWh, for example. The same "derating" for time actually used should then be applied to peaker plants and other time dependent sources. But that may not happen either.

Peaker plants may contribute less to the total power generated because of their intermittent use, but they do contribute unconditionally to the generating capacity because they can generally be turned on at will, to supplement the base load sources.
SMA SB 3000, old BP panels.
• Solar Expert Posts: 62 ✭✭✭✭

The ratio of the actual power produced to the theoretical power produced if a power plant operated at 100% all the time is called the capacity factor.

No power plant will have 100% capacity factor, since everything will occasionally need maintenance. But certainly some base load power plants will be in the 90% and up range.

Solar and wind are going to be a lot worse, more along the lines of 25% and below. However, some fossil fuel plants may be even worse--these would be the "peaking" plants which are expensive to run but can be turned on fast when needed to meet peak loads or if another plant goes offline.

The peaking plants have the advantage of being dispatchable (meaning, the power company can decide when to turn them on or off).

So from the perspective of the power company, wind and solar have the combined disadvantages of low capacity factor and not dispatchable. So from that perspective there seems to be little reason to use wind or solar except for the fact that the fuel cost is zero.

Of course, that ignores the hidden costs of pollution and resource exhaustion from using fossil fuels. The power companies don't (currently) pay these expenses, but society as a whole will have to over time.